Prior Art
In order that fast movements of players can be followed in a large sports field, for example, in watching a soccer or rugby game, or a relatively wide theater stage can be seen from one wing to the other wing, binoculars which have low power but present a wide angle of view are necessary.
As such binoculars of low power, so-called opera glasses have usually been used. While the opera glasses are conveniently compact and lightweight since they comprise so-called Galilean optical system employing a convex lens as an object lens and a concave lens as an eyepiece, it is well known that its relatively dark field of view and relatively small angle of view can not meet the above-described demands.
On the other hand, binoculars with high power generally employ an optical system of inverse reflection comprising a plurality of prisms to obtain a power as high as 7 or higher. However, an actual angle of view presented by such arrangement is limited to the order of 10.degree. and such small angle of view makes it impossible to follow the speedy movement of an object to be viewed.
Although the antinomy in optics that higher the power smaller the visual angle may be unavoidable, it is preferred to employ an object lens having a focal distance as short as possible in order to obtain a wide field of view. However, if a prism is used as an erecting system, a focal distance of the object lens must cover the length of optical path occupied by the prism, and thus the binoculars become correspondingly bulky. In addition, the volume of the prism affects the volumes of both the object lens and the eyepiece, resulting in bulky and weighty binoculars requiring correspondingly high costs of manufacturing.